This invention relates generally to the heating and cooling of objects to be tested and particulatly to the electrical testing of unpackaged integrated circuit chips uncut from their wafers at subfreezing and at high temperatures.
In the production of integrated circuit (IC) devices, it is obviously essential that they be electrically tested prior to their installation as circuit components. Numerous types of circuit test jigs have been developed which will receive various size and types of packaged IC devices and will make the electrical tests necessary to prove that the IC device is properly functional.
While these types of test fixtures may be accurate and provide the proper functional tests, much greater economy is realized by providing the same electrical test to unpackaged circuits that have not yet been cut from the wafer upon which they were formed. The cost of such unpackaged and uncut circuits may be minimal; the cost of cutting an IC from the wafer and packaging it is relatively high.
During the production of integrated circuits, the electrical testing of the unpackaged and uncut chips is accomplished with a prober such as manufactured by Signatone Corporation of Santa Clara, Calif. The prober incorporates a microscope which enables a technician to accurately position a wafer secured to the surface of a vacuum chuck on a movable stage so that the very fine terminal conductors on a selected IC on that wafer may be electrically contacted by hairlike probe tips that extend from adjustable probe positioners, or probe cards, which are coupled to associated electronic test circuitry.
If production quantities of a particular IC are to be tested with a prober, a probe card having a central hole with prepositioned probe tips extending into the hole may be used. The probe tips are arranged so that their tip ends will simultaneously contact all terminal conductors on the IC when the wafer is precisely positioned and the stage is then raised to form the contact between probe tips and IC.
Tests performed with a prober on unpackaged and uncut integrated circuits are often at room temperatures. Tests at elevated temperatures may readily be performed by the use of hollow vacuum chuck by heating the chuck, the wafer, and finally its circuits by passing a fluid at the desired elevated temperature through the chuck or by using an electrical heater buried in the chuck. Unfortunately, very cold tests cannot be readily be performed in the same manner. If chilled fluid is passed through the chuck, the formation of atmospheric moisture on the IC at the dew point will alter the electrical characteristics of the IC, and further cooling will result in frost damages to the IC, fogging of the microscope optics, etc.
Many manufacturers are interested in the characteristics of semiconductor devices at temperature extremes, and many require full electrical testing of every device at such temperature extremes. The invention described herein provides the cooling and/or heating of integrated circuits on a wafer over a wide range of temperatures of between approximately -55.degree. C. to +150.degree. C. or greater. without danger of fogging or frosting and, further, permits rapid temperature changes because the vacuum chuck retaining the wafer is not required to be heated or cooled during the heating and cooling of the wafer.
Briefly described, the invention includes a toroidal chamber or nozzle that is positioned over the central hole and prepositioned probe tips in a probe card. The nozzle has a nitrogen gas inlet coupled through a temperature controllable heater to a source of liquid nitrogen. The downfacing nozzle directs a tubular curtain of the gas down through the probe card hole and upon the surface of a IC wafer located on a thermally insulated stage-mounted vacuum chuck beneath the probe card. The thermal insulation allows the temperature on the wafer to rapidly change without changing the temperature of its surroundings. Some of the dry gas flows laterally under the probe card to purge the atmosphere from the remainder of the wafer and from the prober itself while a portion of the gas is reflected back to the nozzle area to maintain a positive pressure of dry gas and to prevent icing of the upper nozzle surface, the microscope, the probe card, etc. The nozzle contains a temperature sensor for controlling a heater to warm the freezing gas to the temperature desired for its application to the wafer surface.